3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.018 (see, e.g., V11.3.0 of June 2012) describes basic call handling mechanisms. Those call handling mechanisms concern, inter alia, terminating calls directed to Mobile Subscribers (MSs) in a 2nd or 3rd Generation (2G or 3G) communications system such as a Universal Mobile Telecommunications System (UMTS) or a Global System for Mobile Communications (GSM).
An exemplary call handling mechanism for a call originating at a so-called A subscriber and terminating at a roaming MS (also called B subscriber) is shown in FIG. 1 (see section 4.2 of 3GPP TS 23.018). In the scenario illustrated in FIG. 1 the roaming MS is located in a Visited Public Land Mobile Network (VPLMNB).
When a GMSC of the called B subscriber (GMSCB) receives an ISDN User Part (ISUP) Initial Address Message (IAM), it requests routing information from the associated Home Location Register of the B subscriber (NLRB, or simply HLR). To this end, a Send Routing Info (SRI) message is sent to the HLR. Next, the HLR requests a roaming number (i.e., a Mobile Subscriber Roaming Number, MSRN) from the Visitor Location Register currently associated with the B subscriber (VLRB, or simply VLR). In this regard, a Provide Roaming Number (PRN) message is transmitted to the VLR. In a further step the VLR returns the MSRN in a PRN ACK message to the HLR, which forwards the MSRN to the GMSC in an SRI ACK message. The GMSC uses the MSRN to construct an ISUP IAM, which is sent to the Visited MSC of the B subscriber (VMSCB, or simply VMSC).
Upon receipt of the IAM from the GMSC, the VMSC requests information to handle the incoming call from its associated VLR. If the VLR determines that the incoming call is allowed, it requests the VMSC to page the MS. In a next step, the VMSC pages the MS via a Base Station Subsystem associated with the B subscriber (BSSB) using radio interface signalling. In this context, the VMSC starts a local paging timer. When the MS responds, the VMSC informs the VLR in a Page ACK message thereof and stops the paging timer. Next, the VLR instructs the VMSC to connect the call in a Complete Call message, and the VMSC finally establishes a traffic channel to the MS.
The mechanism for handling a terminating call illustrated in FIG. 1 needs to be modified in certain cases, such as Circuit Switched Fallback (CSFB) scenarios. CSFB from Long Term Evolution (LTE) access is specified today in 3GPP TS 23.272.
In brief, CSFB permits in an Evolved Packet System (EPS) the provisioning of voice and other CS domain services (such as services pertaining to Unstructured Supplementary Service Data, or USSD) by re-use of CS infrastructure when the MS (referred to as UE in TS 23.272) is served by the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). A CSFB-enabled MS, connected to E-UTRAN, may thus use 2nd or 3rd Generation (2G or 3G) technologies such as GSM Edge RAN (GERAN) or UTRAN to connect to the CS domain.
FIG. 2 illustrates an exemplary scenario in which a CSFB-enabled MS connected to E-UTRAN (via an LTE-Uu interface) uses GERAN (via a Um interface) or UTRAN (via a Uu interface) to connect to the CS domain in a CSFB situation (see section 4 of 3GPP TS 23.272). CSFB signalling is realized via an SGs interface between a Mobility Management Entity (MME) associated with the MS and a Mobile Switching Centre Server (MSC-S). The MSC-S has to be CSFB-enabled, which basically means that the MSC-S will have to be capable of maintaining SGs associations towards the MME for EPS-attached MSs. A Serving GPRS Support Node (SGSN) is coupled via an S3 interface to the MME and via an optional Gs interface to the MSC-S (not needed for CSFB).
The SGs interface to the MSC-S is used for mobility management and paging procedures between EPS and the CS domain. CSFB is triggered by paging in E-UTRAN and may leads to a Location Update with respect to GERAN or UTRAN (see FIG. 2) in case the MS has changed the Location Area (LA) during CSFB.
CSFB and other roaming services are only available in case E-UTRAN coverage (as defined by Tracking Areas, or TAs) is overlapped by either GERAN or UTRAN coverage (as defined by, for example LAs). A general problem results from the fact that there exists no 1:1 mapping between TAs and LAs.
As shown in FIG. 3, for a terminating call the lacking congruency between TAs and LAs may have the consequence that the MS, when roaming and in particular when falling back from E-UTRAN to GERAN or UTRAN, may land in a LA that is not controlled by the “old” MSC towards which the SGs interface association for the MS has been established by the MME. In such a case the “old” MSC will not be able to terminate the call. For this and other reasons, so-called Mobile Terminating Roaming Retry (MTRR) and Mobile Terminating Roaming Forwarding (MTRF) procedures are defined in Section 5.2 of 3GPP TS 23.018 (see in particular Section 5.2.3) to allow that call termination is tried again towards the “new” MSC controlling the cell the MS is presently camping on. The new MSC will assign a “new” MSRN to the MS that is different from the “old” MSRN assigned by the old MSC.